1. Field of the Invention
The present invention generally relates to extrusion-blow molding apparatus, and particularly to apparatus for molding plastic can bodies suitable for joining to metal can ends.
2. Description of the Prior Art
There are a number of processes available for making a plastic container having a wide-mouth suitable for combination with a metal can end using a typical double rolled seam. Wide-mouth stretch blow-molded PET containers are known that are designed to hold a pressurized contents, such as tennis balls. In such containers, it is desirable to have a high biaxial molecular oriented flange of desired strength and thickness extending radially outwardly from around the wide open mouth for receiving a metal end closure. Examples of such containers are disclosed in U.S. Pat. Nos. 4,496,064 and 4,576,843, which require complete biaxial molecular orientation for adequate performance. The patents disclose the formation of an intermediate article utilizing an oblique truncated cone type of accommodation area, including the flange portion surrounding the wide open mouth of the tubular body. The truncated cone shape imparts a gradual increase in biaxial molecular orientation from a neck area at the top to the flange portion at the bottom of the accommodation portion. The accommodation portion is then removed by cutting through the flange to provide a container having a flange around the open mouth with the desired biaxial molecular orientation. However, the design of the accommodation portion in the form of the oblique truncated cone makes it difficult to consistently control the degree of biaxial molecular orientation and the thickness of the flange material which often results in undesirable strength and thickness variations in the flange for receiving a metal end closure by double seaming operations.
Another example is disclosed in U.S. Pat. No. 4,894,268 in which an injection molded preform is blow molded into an intermediate article within a cavity having a flange forming feature in a region of the blown intermediate article that sufficient biaxial orientation of the polymer will take place. The flange extends around and radially outwardly from an upper open end of a tubular body portion of the intermediate wide mouth container article. The flange is a portion of an accommodation portion in the intermediate article which extends from the upper end of the tubular body portion and includes a flange of high biaxial molecular orientation. The flange is adapted to be annularly cut to remove the remainder of the accommodation portion during fabrication of the wide-mouth container from the intermediate article. A stepped wall portion of decreasing biaxial molecular orientation extends around and upwardly and inwardly from the flange and includes at least two successive upwardly and inwardly extending step means or areas for collecting a quantity of PET during blow-molding and for increasing biaxial molecular orientation and controlling the thickness of the flange
More recently, U.S. Pat. No. 5,342,663 discloses the formation of a preform with an injection molded flange, which in subsequent molding operations, is not subjected to any biaxial stretch during the blow molding process performed on the remainder of the preform, thus maintaining its injected profile. The flange is said to have sufficient mechanical durability and non-gas-permeability to endure any subsequent double-seaming operation due to a flow orientation of plastic molding material, which is made to flow through a relatively narrow area. The flange is said to be precisely dimensioned, having mechanical durability to be double-seamed with a lid. The flange is disclosed to be preferably 0.3 mm to 0.7 mm thick.
The continued recognition of the importance of maintaining the precise dimensional uniformity of the flange, and the difficulty of achieving that uniformity in extrusion-blow molding processes, is reflected in U.S. Pat. No. 7,153,446, which disclosed an apparatus and process for molding a flange suitable for double seaming with a metal lid. The mold includes a circumferential groove that is filled with flowable thermoplastic material when the material is inflated in the mold. A localized region of the mold adjacent the groove is heated by a heating system to a higher temperature than other portions of the mold to render the thermoplastic less viscous so that the groove is completely filled. The heating system can be of various types, including electrical heaters, circulated hot fluid heaters, etc., and is locally disposed in the vicinity of the groove in the mold for heating the mold surfaces in the groove. The mold can also include a cooling system locally disposed in the vicinity of the groove for cooling the flange of the blow-molded article just prior to opening the mold and removing the article. The cooling system can comprise, for example, a circulated coolant system or the like. The manufacture of molds having such localized groove focused heating and cooling systems is expected to be more expensive than conventional molds having uniform heating or cooling systems. Additional controls are also required to control the localized groove focused heating and cooling systems beyond that conventionally required.
Thus, there remains a need for an extrusion blow mold capable of reliably forming a plastic bottle that includes a radial flange suitable for roll-form attachment to a metal can end that can be simply constructed and operated without extraordinary heating and control features.